The present invention relates to an x-ray computed tomography (CT) method and apparatus, and more particularly, to an x-ray computed tomography method and apparatus which is arranged to contain image data pixel value ranges for several slices within the same preferred range.
In conventional x-ray computed tomography apparatuses, a fan-shaped x-ray beam emitted from an x-ray source impinges upon a subject, and the transmitted x-rays are measured by a one-dimensional array of x-ray detector elements consisting of a plurality of detector elements arranged along the extent of the fan-shaped x-ray beam.
The transmitted x-rays are measured in a plurality of view directions while the x-ray source and the detector element array are rotated around the subject. This procedure for measuring the transmitted x-rays is referred to as "scanning". Based on the measured data for the plurality of views acquired by the scanning, a tomographic image of the subject is reconstructed.
When image data of the tomographic image of the subject thus produced is displayed on a image display device such as a CRT (cathode ray tube) display, pixel values in the image data (i.e., CT values in the case of the x-ray computed tomography apparatus) are determined according to the transmission factor of the x-ray, and generally defined such that the air has a value of -1,000 and the water has a value of 0.
The image is displayed by converting such image data pixel values into display data having gradations of the order of 256. Although the number of the gradations of the display data varies with the image display circuit or circuit configuration of the CRT display device, it is generally of the order of 256 (the intensity data is processed in 8-bit), and it is not practical to provide a wide range of gradation for display, such as those having the image data pixel value ranging from -1,000 to +2,000.
For example, consider the case in which the internal organs of the subject to be observed give the pixel value ranging between -150 and +150 in CT value. In this case, the concept of "window width" is introduced. The display data is converted to display a range corresponding to the window width with 256 gradations. The image is displayed with shading, in which the portion having a CT value above the upper limit is represented in white or black and the portion having a CT value below the lower limit is represented in black or white. This procedure can adapt the image data of the region which is to be observed to be contained within the displayable gradation range, and the image data can be displayed as varying contrast.
Prior to the tomographic imaging, scout scanning is sometimes performed to determine preferred imaging positions. In the scout scanning, while translating the table plate on which the subject is rested with the x-ray tube and the x-ray detector fixed, the subject is irradiated with x-rays to obtain an x-ray visualized image.
Japanese Patent Application Laid Open No. 1-293844 (1989) discloses a technique in which an x-ray dose as a scanning condition is determined by estimating the x-ray attenuation relative to the average detected value of the transmitted x-rays during the scout scanning.
Since the body shapes of several subjects are unequal, however, it is difficult to determine the optimum scanning condition from the detected value in the scout scanning performed at a single angular position of the x-ray source.
Moreover, since the cross-sectional shape of a subject varies from slice to slice, the dispersion in pixel value range of the obtained image data occurs between the slices. This poses a problem that the respective pixel value ranges of the image data acquired from several slices are different from slice to slice.